Question

The pressure on a sample of water at its triple point is reduced while the temperature is held constant. Which phases changes are favoured? (I) melting of ice (II) sublimation of ice (III) vaporization of liquid water (a) I only (b) III only (c) II only (d) II and III

Short answer

Phase changes (II) sublimation of ice and (III) vaporization of liquid water are favored.

Step-by-step solution

- Understand the Triple Point

The triple point of water is the temperature and pressure at which water coexists in equilibrium in all three phases: solid (ice), liquid, and gas (vapor). At the triple point, the slightest change in pressure or temperature can shift the equilibrium, causing a phase change.

- Analyzing the Effect of Pressure Reduction at Constant Temperature

When the pressure is reduced at a constant temperature, the water system tries to re-establish equilibrium by favoring the phases that occupy more volume, since decrease in pressure allows the molecules to spread out more. Under this condition, phases that have lower density (and thus higher volume) are favored.

- Determining Favored Phase Changes

Reducing pressure at a constant temperature will favor the phase change from solid to gas (sublimation) and from liquid to gas (vaporization) because both result in a greater volume per unit of water. Melting of ice is not favored because the liquid phase has a higher density than solid, so it occupies less volume compared to the gas phase.

Key concepts

Phase Changes in Matter

Understanding the phase changes in matter is fundamental to grasping the behavior of water at its triple point. Matter can exist in different phases: solid, liquid, and gas.

Each phase is characterized by its unique set of physical properties, such as density, volume, and molecular arrangement. A change in either temperature or pressure can induce a transition from one phase to another. These transitions include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid).

In the provided exercise, the reduction of pressure at a constant temperature can lead to a phase change. Specifically, reductions in pressure generally favor phase transitions to a less dense phase, because less pressure allows molecules to move farther apart, increasing volume. It's essential when studying phase changes to pay attention to the effects of both temperature and pressure, as they can dramatically alter the equilibrium of a substance’s state.

Sublimation of Ice

Sublimation is the phase transformation where a solid directly converts into gas without passing through the intermediate liquid phase. In the case of ice, sublimation occurs when its molecules gain enough energy to break free from the solid structure and disperse into the air as water vapor.

At the triple point, ice can sublimate at a specific equilibrium pressure and temperature. If the pressure is decreased while maintaining the same temperature, sublimation of ice is favored over melting. This is because lower pressure allows the water molecules in ice to spread out more and become gas, thus resulting in a greater volume, which is favored in low-pressure environments. This concept helps students to understand and visualize how molecular interactions are influenced by external conditions like pressure.

The sublimation of ice is a key example of a phase change influenced by pressure, shown vividly in environments like freeze-drying, where the sublimation process is used to preserve food and other perishables.

Vaporization of Liquid Water

Vaporization is the process where liquid water changes into water vapor, which is a gas. This phase change can take place in two forms: evaporation and boiling. Evaporation occurs at temperatures below the boiling point, and it can happen on the liquid’s surface. Boiling, on the other hand, involves the formation of vapor bubbles within the liquid when its temperature reaches the boiling point.

When the pressure is reduced at the water’s triple point temperature, the tendency for liquid water to move into a gaseous state increases. This is because the molecules require less energy to escape the liquid phase under lower pressure, facilitating vaporization. In a low-pressure environment, the liquid phase is less favored compared to gas, because the vapor phase has a lower density and will occupy a larger volume. This understanding of vaporization underpins many natural and industrial processes, from weather patterns involving cloud formation to the steam engines that powered the Industrial Revolution.